1. Field of the Invention
The present invention relates to a method of manufacturing a wiring board including a manufacturing step of forming an insulating layer having a flat upper surface on a wiring layer having a wiring pattern, a composite sheet for manufacturing a wiring board to be used therein, and a wiring board which can be obtained by the manufacturing method. The present invention is prefer for a method of manufacturing a multilayer wiring board.
2. Description of the Related Art
In the steps of forming a core substrate, an insulating layer or the like of a printed wiring board to be used in electronic equipment or the like, conventionally, there has been used a prepreg in which a glass fiber fabric or a polymer nonwoven fabric is impregnated with a thermosetting resin and is half cured. The prepreg is thermally pressed together with copper foils provided on both surfaces, for example, so that a double-sided copper foils laminated plate can be obtained, and a wiring pattern is formed on the copper foil and can be thus used as the core substrate of a multilayer wiring board.
In recent years, the wiring board has a finer wiring pattern because of the high integration of a wiring, and furthermore, a multilayer structure is achieved and each layer is thinned. For this reason, in the case in which a glass fiber fabric impregnated with an epoxy resin is used for the prepreg, there is a problem in that drilling is carried out with difficulty due to a glass fiber when laser via processing is to be carried out. Therefore, there have been proposed various prepregs using, for a reinforcing phase, a polymer nonwoven fabric on which the laser via processing can be carried out more easily.
When an insulating layer is formed on a wiring layer having a wiring pattern by using the prepreg having the polymer nonwoven fabric for the reinforcing phase, however, a flat insulating layer is formed with difficulty due to the unevenness of the wiring pattern. And a wiring layer to be an upper layer is not flat. Consequently, there is a problem in that the step of providing an upper layer to be carried out subsequently is hindered. For this reason, in a multilayer wiring board having each layer thinned, there have been particularly employed a method of applying a thermosetting resin in place of the provision of a prepreg and a method of providing a thermosetting resin sheet to form an insulating layer. In these methods, since the reinforcing phase is not present, it is hard to control a gap for forming an insulating layer. Thus, the methods are not advantageous in respect of mass production.
In place of the polymer nonwoven fabric, there has also been known a prepreg using a porous film formed of aromatic polyamide (Japanese Laid-Open Patent Publication No. Hei 9-324060). Such a prepreg is usually used for manufacturing a double-sided copper foil laminated plate having a copper foil integrated on both surfaces thereof and is used for fabricating a core substrate.
Therefore, it is an object of the present invention to provide a method of manufacturing a wiring board which is advantageous to mass production because an insulating layer having a flat upper surface can be formed on a wiring layer, and furthermore, a gap for forming the insulating layer can be controlled easily, a composite sheet for manufacturing a wiring board to be used for the method, and a wiring board which can be obtained by the same method.